Radiused mold walls for plastic tubs

ABSTRACT

The impact resistance of a one piece molded plastic tub along necessary tub wall discontinuities is increased by preforming such discontinuities along a radius to decrease the notch effect characteristic of an abrupt discontinuity.

United States Patent Clark Oct. 29, 1974 [54] RADIUSED MOLD WALLS FOR PLASTIC 3,009,603 ll/ I961 Stockdale 220/94 X TUBS 3,307,729 3/1967 Schwartz 220/97 R X 3,327,896 6/1967 Asenbauer 220/73 X inventor: David Clark, 7304 Cross 3,377,990 4/1968 Mitchell 220/71 x Louisville, Ky. 40228 3,438,544 4/!969 Cloyd 220/71 X [22] Filed: 1972 Primary ExaminerWilliam 1. Price [21] App]. No; 319,202 Assistant Examiner-Steven M. Pollard Attorney, Agent, or Firm-Francis H. Boos [52] US. Cl 220/72, 220/83, 220/66 511 Int. Cl B65d 7/42 [571 ABSTRACT [58] Field of Search 220/63, 73, 72, 71, 74, Th impa resistance of a one piece molded plastic 220/83, 97, 66; 134/186, 200, 20l tub along necessary tub wall discontinuities is increased by preforming such discontinuities along a ra- [56] References Cited dius to decrease the notch effect characteristic of an UNITED STATES PATENTS abrupt dlscommulw 2,340,024 1/1944 Skaller 220/97 A X 2 Claims, 8 Drawing Figures 7 1 RADIUSED MOLD WALLS FOR PLASTIC TUBS BACKGROUND OF THE INVENTION The invention is directed to large capacity front loading dishwashers of the type normally comprising a permanent installation and employing upper and lower dish supporting racks as opposed to the smaller, low capacity, portable and/or top loading dishwashers. The reason for the distinction is that the ultimate objective of the invention is the employment of a unipartite plastic tub in large capacity, front loading dishwashers and while some of the problems presented by a proposed use of such a tub would be common to dishwashers of either type; the majority are distinct. Typical distinctions relate to increasing wall flexibility with increasing area, the absence of encompassing metal housings,

greater tub shape complexity and additional interior tub mountings such as dual rack support structures. Accordingly, the ensuing background discussion relates primarily to front loading dishwashers of the type normally adapted for a permanent, under cabinet, installation.

The high temperature, corrosive environment to which automatic dishwasher chambers are subjected dictates that the surfaces of those internal components in contact with such environment be either highly resistant, or immune, to corrosion. Corrosion resistant met als such as stainless steel represent a substantial cost factor in this highly competitive field and the long term industry trend has been toward the use of inert materials to resist the corrosive effects of the chamber environment which are magnified adjacent the usual metal tub seam welds. Insofar as the tub itself, representing the far greater internal surface area exposure, is concerned; the general trend in the United States has been toward the application of an adherent plastic coating to the tub interior while European manufacturers have gone more to the use of plastic tub liners. In either event, the basic structural and design parameters that have prevailed since the advent of the modern front loading automatic dishwasher remained valid. This for the reason that overall dishwasher design parameters, both as regards utility and structural integrity, are virtually unaffected by whether the conventional metal tub is coated, lined or exposed.

Conceptually, the use of a unipartite plastic dish washer tub, i.e., one not requiring a more rigid backing or outer support structure, represents a substantial advance in the art. Exemplary are the greatly decreased cost factors of a molded plastic tub vis-a-vis that of a metal tub requiring a further coating or lining operation; reduction in finishing labor requirements and particularly tub welding operations; longer tub life due to corrosion resistance which is independent of faulty coating procedures or chipping; decreased thermal and acoustical transmission thus minimizing need for separate insulation; and a lesser inventory requirement for separable components susceptible of integral molding with the tub.

The reduction to practice of such a concept, as applied to front loading dishwashers on an assembly line basis, however, presents a myriad of problems which arise as a requirement for different approaches to those engineering considerations which had previously been predicated upon the use of a metal tub. The distinctions are primarily those associated with material strength. the unusually high temperature environment to which the tub is subjected and considerations relating to plasconventional front loading tub is more than sufficient to resist excessive wall deformation as a function of normal loading forces as during installation, adjustment or use and temperature variations from room ambient to approximately 255F.; such normally applied loading forces would, in the absence of appropriate compensating structure, produce unacceptable wall deformation in a like configured plastic tub. The use of strandlike reinforcing fillers such as fiber glass would, seemingly, ameliorate the overall problem of wall deformation. In actuality, however, the presence of such reinforcing fibers not only effect tub discoloration as discussed below, but their random distribution precludes an accurate prediction of thermal growth characteristics which is an essential parameter in the design of certain compensating structure. Similarly, impact strength is normally of little moment in the design and installation of metallic tubs but is a matter of major concern in plastic tub design. Furthermore, various structural features appearing in a formed plastic tub as an incident to the forming operation create problems not preveiously associated with metal tub fabrication. Exemplary of the latter distinction are the presence, on the finished product, of mold lines, mold surfaces and/or mold retention ribs creating localized areas of decreased impact resistance as well as the usual wall draft to permit removal of the product, or tub, from the forming mold. One primary disadvantage arising from this necessary wall draft concerns the mounting of the upper dishwasher rack supporting tracks in parallel.

In addition to the foregoing considerations which relate primarily to mounting and/or assembly procedures for large capacity front loading machines, the desirability of precluding tub discoloration and the desirability for firewall separation of the tub and those electrical components constituting a fire hazard are common to all automatic dishwashing machines. As regards plastic tub discoloration, it is preferred to avoid the use of certain conventional strand type reinforcing fillers, such as chopped strand fiber glass, for the reason that their intersection with the interior plastic surface may provide a bleed path for food stains to permeate the tub structure.

The prior art suggestions relating to the use of small capacity plastic tubs of the top loading type, as in US. Pat. No. 2,691,986 and the use of separate plastic tub components, as in US. Pats. No. 2,877,778 and No. 3,385,306 offer little in the way of guidelines as regards a practical reduction to practice of a large capacity front loading tub for the reason that most of the problems requiring solution are not present in smaller and- /or top loading units and particularly those which employ an enveloping metal housing.

The present invention is concerned with the problem of impact resistance as deriving from the necessary presence of mold lines and/0r surfaces on the one piece molded tub walls.

It is apparent that if the advantages in the elimination of the conventional front loading metal tub and the substitution therefor of a plastic tub, as regards decreased cost and weight, are to be retained the necessary mounting and support structure must not, in effect, comprise a confining tub support housing. Rather, a purpose of the invention is to provide an operative,

front loading dishwasher unit having a plastic tub whose outer walls comprise the great majority of the outer dishwasher unit construction and would actually be exposed to view prior to installation of the dishwasher unit within an outer decorative housing, which decorative housing may comprise a free standing, portable unit or apermanent undercabinet installation. Accordingly, it is necessary that the tub, whose exemplary wall thickness is less than 0.2 inches, have a certain minimal impact resistance for factory handling and in use which latter resistance is prescribed by Underwriters Laboratory to be foot pounds.

SUMMARY OF THE INVENTION In the use of a multi-part contractible mold core for the formation of a unipartite plastic tub having a complexly configured bottom wall; the core must collapse away from the lower or complexly configured wall to permit withdrawal of the molded part from the core. Relative movement between the core parts thus rakes place along another of the tub walls which, in the case of the instant invention, is the rear wall remote from the open tub front. Since it is essential that the high pressure plastic injection not be permitted to enter between the core sections; the lines of juncture between the core sections which lie along the mold surfaces are normally formed as razor sharp edges to insure a close fit between the parts. In the particular three-part mold core and tub design herein discussed, the upper and lower mold core sections undergo vertical movement relative to each other and the central core section and rear tub wall so that the lower mold section may clear the complexly configured bottom wall for part removal. During the course of the contracting mold core movement, the sharp edges would, if not compensated for, score the rear tub wall. One solution to the problem is to have the central core section extend just slightly beyond the upper and lower core sections so that their vertical movement, upon core contraction, would be spaced from the rear tub wall portion formed against the central core member as will be more easily understood from the ensuing discussion and drawings. It is this latter concept, as applied to include the formation of radiused mold walls to provide that necessary impact resistance that would be lacking in the absence of such radiused mold walls, to which the present invention is directed.

DESCRIPTION OF THE DRAWINGS FIG. I is a rear perspective of a front loading dishwasher employing a plastic tub;

FIG. 2 is a sectional view taken along line 22 of FIG. 1;

FIG. 3 is a schematic illustration of a tub forming mold wherein the end faces of the various contractible core sections terminate in a common plane;

FIG. 4 is a view similar to FIG. 3 but wherein the end face of the central core section extends slightly beyond those of the upper and lower core sections;

FIG. 5 is a schematic illustration of the relative contracting movement undergone by the various core sections of FIG. 4;

FIG. 6 is a diagrammatic illustration of a conven' tional notch test;

FIG. 7 is a fragmentary schematic of the particular contractible core configuration used in the formation of the tub of FIG. I; and

FIG. 8 is a diagrammatic illustration of the application of a test, similar to that of FIG. 6, as applied to a semicircular cutout.

DESCRIPTION OF THE PREFERRED EMBODIMENT In FIG. 1 is illustrated a front loading dishwasher 10 whose plastic tub 12 was formed as a one piece injection molding and whose generally planar rear wall 14 is disrupted by arcuate mold walls 16 continuous with and merging an offset planar wall portion 18 with the remainder of the rear wall.

The reason for the presence of the offset wall portion 18 will become more apparent from the following discussion relating to the one piece molding procedure used in the formation of tub 12.

The lower tub wall 20 is necessarily formed with a complex configuration to define the necessary service openings and front reservoir forming ledge 22. In line with conventional practice, a multi-part contractible mold core is used to permit withdrawal of the finished tub from the mold core. A schematic illustration of the general type mold elements which would be employed appears in FIG. 3 and comprises a multipartite female mold 24 and three-part, contractible mold core 26 having upper and lower core sections 28, 30 wedgingly mated with central core section 32. In its simplest form, the one piece formation of a molded tub part 34 involves the injection of plastic material, such as by injection opening 36, into the mold cavity defined between female mold 24 and male core 26. Thereafter, the core sections 28, 30, 32 undergo conventional, relative contracting movement in the directions indicated by arrows 38, 40, 42, respectively. The upper and lower core sections 28, 30 thus collapse vertically and clear the upper and lower tub walls from interferring engagement with their respective forming core sections upon horizontal removal of the part. The outermost ends of the upper and lower core wedge surfaces 44, 46 terminate in sharp edges 48, 50 to provide a close fit with the central core section to prevent the entry of injected plastic therebetween. If the outer end faces 52, 54, 56 of the upper, central and lower core sections lay in a single vertical plane perpendicular to the plane of the drawing, as in FIG. 3, the subsequent vertical movement of the upper and lower core sections would result in scoring of the rear tub wall by the vertically moving sharp edges 48, 50.

The problem of scoring may be eliminated by dimensioning the central core member to include an end face 58 extending just beyond the plane of the upper and lower core section end faces 52', 56' as illustrated in FIG. 4. This mold core construction results in the formation of mold lines 60 defining a recessed inner wall surface 62 which lies outside the vertical plane of movement of the sharp edges 48', 50' so that scoring of the rear tub wall, upon contracting movement of the core sections as in FIG. 5, is avoided.

It was first assumed that the extension of the central core section end face beyond the plane of the upper and lower core end faces should be as small as consistent with the purpose of providing clearance between the vertically moving edges 48', 50' and the offset inner wall surface 62, as on the order of 0.010 inches; and that no design changes other than the new dimension need be considered. It was found, however, that the tub construction produced by such a mold core construction exhibited a very low impact resistance along the generally perpendicularly related mold lines 60 thus created. Stated differently, the sharp discontinuity in the tub wall exhibited the well known notch effect upon impact.

Comparative, schematic illustrations of the conventional notch effect as applied to a plastic tub wall and the basis for that tubwall configuration constituting the present invention appear in FIGS. 6 and 8. Thus if a notched bar 64 (FIG. 6) be fixed at opposite ends thereof and an impact applied opposite notch 66, the applied stress is concentrated substantially along a line between the point of impact and the notch apex 68. This notch effect is exhibited in a similar manner when the notch is, in effect, a reentrant portion of a cutaway section such as would result if the bar were cut away along phantom line 70. This, of course, is the precise tub wall configuration shown in FIG. 5 resulting from what would appear to be a logical solution to the scoring problem and which configuration exhibits the notch effect, just discussed.

Conversely, if an impact be applied opposite a semicircular cutout 72 (FIG. 8) in a similarly fixed bar 74, the stress concentration is spread over the cutout area and, graphically, would yield a stress gradient curve decreasing on either side of the lowest point of the cutout 72. If a portion 76 of bar 74 then be cut away it will be seen that stress concentration is still spread over a substantial area as compared with the virtual line concentration resulting from the notch configuration of FIG. 6. This concept of spreading stress concentration is extrapolated, in accordance with the present invention, by forming the cutout portion along a substantial radius as on the order of H16 inch-fit inch and maintaining overall wall thickness by recessing the female mold. Thus, the phantom line represention in FIG. 8 corresponds to the offset wall portion 18 (FIG. 1).

The particular mold core configuration used to form the tub I2 is schematically illustrated in FIG. 7 and differs from those of FIGS. 3-5 in that the end face 78 of central core section 80 extends substantially beyond the plane of the upper and lower core section end faces, as on the order of 0.15 inch-0.25 inch, and whose upper and lower edges 82, 84 are radiused. The female mold 86 is formed with a corresponding recess 88 to maintain constant wall thickness.

It has been found that, with the particular polypropylene based plastic and exemplary wall thickness herein disclosed, arcuate mold walls 16 formed in accordance with the exemplary parameters herein discussed exhibit resistance to impact in excess of 5 foot pounds. Such parameters are, obviously, varied in accordance with the principles herein discussed to accommodate different compositions, wall thicknesses and desired resistance to impact; the criteria being to provide an offset wall portion 18 whose inner surface 90 lies beyond,the vertical plane of movement of core section edges 48", 50" and whose defining mold walls are radiused to the extent necessary to maintain desired impact resistance.

While the background discussion of this specification is related primarily to front-loading dishwashers of the type normally adapted for a permanent undercounter installation as opposed to the type of dishwasher having a hinged top closure for top-loading, it should be emphasized that many such machines are commercially produced and purchased for indefinite use as a freestanding model adaptable at a later time by the owner to a built-in unit beneath a kitchen countertop to blend with the matching cabinet structure of the kitchen. Generally, the difference between the unit designed as a convertible" and the unit to be immediately permanently installed is the inclusion of side, rear and top cabinet appearance panels to the basic unit structure as designed for permanent installation. Therefore, it should be understood that the present invention is specifically directed to the front-loading type of dish washer structure whether of the generally lower-cost unit having no appearance cabinet enclosure or the more elaborate, temporarily portable unit designed for later modification for use in a fixed location.

I claim:

1. A unipartite plastic dishwasher tub having an open front, a compound lower wall configuration and a rear wall, said rear wall having a generally planar configuration disrupted by an offset parallel planar wall portion merged into said rear wall by radiused merging offset portions, which portions are radiused to a dimension in the range of about 1/16 to about A inch and are of a constant wall thickness, thereby giving said back wall a resistance to impact in excess of 5 foot-pounds.

2. A unipartite plastic dishwasher tub having an open front, a compound lower wall configuration and a rear wall, said rear wall having a generally planar configuration disrupted by an offset parallel planar wall portion merged into said rear wall by radiused merging offset portions, which portions are radiused to a dimension in the range of about l/l6 to about 1 inch and are of a constant wall thickness, thereby giving said back wall a resistance to impact in excess of 5 foot-pounds, said radiused merging offset portions being the result of injection molding wherein first and second core sections are wedgingly mated with a central core section inside a multipartite female mold section having a recessed portion to define a mold cavity, said first and second core sections having outermost ends which terminate in coplanar end faces and sharp edges to provide a close fit with said central core section, said central core section having an end face which extends substantially beyond the plane of said first and second core section end faces in juxtaposition with said recessed portion of said female mold section, said extending end face also having edges radiused to a dimension in the range of about l/ 16 to A inch to cooperate with said recessed portion of said female mold section, which is bounded by edges radiused to a dimension in the range of between about H16 and A. inch to provide for the formation of said radiused merging offset portions in said tub rear wall. 

1. A unipartite plastic dishwasher tub having an open front, a compound lower wall configuration and a rear wall, said rear wall having a generally planar configuration disrupted by an offset parallel planar wall portion merged into said rear wall by radiused merging offset portions, which portions are radiused to a dimension in the range of about 1/16 to about 1/4 inch and are of a constant wall thickness, thereby giving said back wall a resistance to impact in excess of 5 foot-pounds.
 2. A unipartite plastic dishwasher tub having an open front, a compound lower wall configuration and a rear wall, said rear wall having a generally planar configuration disrupted by an offset parallel planar wall portion merged into said rear wall by radiused merging offset portions, which portions are radiused to a dimension in the range of about 1/16 to about 1/4 inch and are of a constant wall thickness, thereby giving said back wall a resistance to impact in excess of 5 foot-pounds, said radiused merging offset portions being the result of injection molding wherein first and second core sections are wedgingly mated with a central Core section inside a multipartite female mold section having a recessed portion to define a mold cavity, said first and second core sections having outermost ends which terminate in coplanar end faces and sharp edges to provide a close fit with said central core section, said central core section having an end face which extends substantially beyond the plane of said first and second core section end faces in juxtaposition with said recessed portion of said female mold section, said extending end face also having edges radiused to a dimension in the range of about 1/16 to 1/4 inch to cooperate with said recessed portion of said female mold section, which is bounded by edges radiused to a dimension in the range of between about 1/16 and 1/4 inch to provide for the formation of said radiused merging offset portions in said tub rear wall. 